1. Field of the Invention
The present invention relates generally to fluid propulsion systems. More particularly, the invention concerns a novel thrust enhancing propeller guard assembly for use in connection with outboard motors.
2. Discussion of the Prior Art
Rotating propellers are an integral aspect of outboard motors of the character used to propel various types of small watercraft. Unguarded rotating propellers present a substantial hazard to submerged objects and to people in the water being traversed by the watercraft. Therefore, there is a need for propeller guard that not only guards against damage caused by the rotating propellers of an outboard motor, but also preferably enhances the propulsion characteristics of the motor. As will be better understood from the description that follows, the novel propeller guard assembly of the present invention accomplishes both of these desired results. Additionally the unique propeller guard assembly of the present invention increases fuel efficiency, stabilizes boat handling and also functions to protect the propeller from damage caused by submerged objects.
A number of different types of propeller guards have been suggested in the past. One such apparatus is disclosed in U.S. Pat. No. 4,637,801 issued to Schultz. The propeller guard disclosed in the aforementioned patent comprises a primary cowling mounted on the motor housing coaxially with the axis of rotation of the propeller, at least a portion of the primary cowling extending forward of the propeller and beyond the outboard motor where a secondary cowling coaxially mounted with respect to the primary cowling and spaced therefrom in an overlapping relationship therewith so that the leading edge of the secondary cowling extends over the trailing edge of the primary cowling and the trailing edge of the secondary cowling extends rearwardly beyond the outboard motor propeller. The apparatus also includes a plurality of axial vanes secured to the primary cowling at spaced annular positions around the outer surface of the primary cowling and secured to the inner surface of the secondary cowling to hold the cowlings in spaced coaxial relationship.
Another prior art propeller guard arrangement is disclosed in U.S. Pat. No. 5,651,707 issued to Lemont. This patent discloses a low-aspect ratio propeller system provided with a multiple ring structure formed with a plurality of circular or noncircular, annular, narrow equivalent Air foil rings which are held by rails in a predetermined relationship with the propeller blades. Still another prior art U.S. Pat. No. 4,957,459 issued to Snyder discloses a propeller shroud comprising a cage around the motor gear case and the propeller that includes a pair of inner spokes extending inwardly and bearing against the junction of the motor or skeg and the torpedo shaped central portion of the motor such that impact force on the cage is transmitted to the junction. The cage has a first portion with the leading edge extending along the front of the skeg and tapering rearwardly and outwardly and a rear generally cylindrical portion around the propeller and extending rearwardly from the front portion.
It is a primary object of the present invention to provide a propeller guard assembly for water craft that, in cooperation with a rotating propeller, creates a substantial increase in propulsive thrust and provides improved all around shielding of the latest state of the art propeller configurations such as wider propeller blades and contra rotating dual propeller systems with longer axial dimensions.
Another object of the invention is to provide a propeller guard of the aforementioned character, which can be easily attached to the shaft housing of present-day outboard motors, or to the stem drive of present-day inboard motors.
Another object of the invention is to provide an improved propeller guard as described in the preceding paragraphs that can be fabricated from inexpensive resilient material.
As will be better understood from the description that follows, the increase in propulsive thrust of the propeller guard assembly of the invention is created primarily by the angle of the attack and hydrofoil cross section of the four guard rings or cowlings of the device. Each guard ring has the same dynamic flow characteristics as a round airfoil wing adopted originally by the kort nozzle.
As secondary reason for the increase in propulsive thrust of the propeller guard assembly of the invention resides in the fact that the thrust realized is equal to the ratio between the effective projected propeller area and the area of trailing edges of the large primary and secondary guard rings which decreases the tendency of the propeller race to spread, particularly at high propeller slip during acceleration and take off which is advantageous when the pleasure boat is used for water skiing purposes.